The synthesis of various multi-arm star polymers has long been of growing practical and theoretical interest to a variety of industries. Such star polymers have shown to be useful as, inter alia, surfactants, lubricants, rheology modifiers, and viscosity modifiers. In fact, star polymers are now considered by many to be state-of-the-art viscosity modifiers and oil additives, although the potential of many of these star polymers for these applications is still being evaluated and tested.
One well-known representative of this class of materials currently being used as an oil additive is commercially available from the Shell Oil Co. (Houston, Tex.) under the tradename Shelvis. This oil additive is a multi-arm star molecule consisting of many hydrogenated polyisoprene arms emanating from an ill-defined, single core of crosslinked polydivinylbenzene (PDVB). By the term "ill-defined" it is meant that the core of the star polymer, e.g., PDVB, is an uncontrolled, crosslinked, gel-like structure having unsaturation sites in the core. In comparison, "well-defined" cores are built of readily characterizable, soluble molecules which are precursors to the core. As a result, the structure of the resultant star polymers having well-defined cores can be controlled.
Also, it is believed that the resultant star polymers having well-defined cores may impart better shear stability than star polymers using ill-defined cores. That is, the presence of unsaturation sites (i.e., double bonds) in the ill-defined cores (PDVB) provides for the possibility that the resultant star polymers will be less shear stable and more sensitive to oxidative reactions than the star polymers having well-defined cores. Thus, in engine oil where shear stability is of critical importance, the possibility exists that during high temperature use and heavy shear in the engine, the ill-defined cores will degrade.
Similarly, the polyisoprene arms in the Shelvis product may contain some unsaturation which is also undesirable. Like the core, the presence of double bonds in the arms may cause them to decompose as well during high temperature and heavy shear conditions within the engine.
Recently, there has been a growing interest in star polymers consisting of multiple polyisobutylene (PIB) arms. For example, Kennedy et al. U.S. Pat. No. 5,395,885 describes the synthesis of star polymers having multiple PIB arms and PDVB cores using cationic synthesis techniques. Because the structure of polyisobutylene is readily characterized and contains no unsaturation, these PIB-based stars are suspected to be useful for a variety of applications such as motor oil additives and viscosity index improvers. However, their potential is still being evaluated and tested, and in motor oil additives where shear stability is of critical importance, the possibility remains that, because of the use of ill-defined, crosslinked aromatic cores such as PDVB, the PIB-PDVB stars currently being tested may not be highly desirable for such use.
On the other hand, studies have demonstrated that silicone oils apparently have superior shear stability properties as compared to hydrocarbon oils. For example, Fitzsimmons et al., in Trans. ASME, 68, 361 (1946), have shown that the viscosity of a silicone oil decreased less than 2 percent after 105,000 cycles, whereas the viscosity of a hydrocarbon oil dropped by more than 50 percent after only 18,000 cycles under certain operating conditions in an aircraft gear pump. Thus, it is seen as highly desirable to provide a star polymer having multiple well-defined polyisobutylene arms and a well-defined, silicone-based core which polymer would be shear-stable.
To that end, attempts have been made to provide such potentially useful star polymers. In pending U.S. application Ser. No. 08/620,421 (allowed), the inventors of record demonstrated the first synthesis of well-defined, first order stars having multiple arms of polyisobutylene emanating from a cyclosiloxane core. These first order stars were formed by hydrosilation of, inter alia, .omega.-allyl-terminated polyisobutylene (PIB--CC.dbd.C) with methylcyclosiloxanes carrying 4 to 8 SiH groups (D.sub.n.sup.H, where n=4 to 8).
By the term "first order" it is meant that these star polymers are essentially being synthesized by linking, via hydrosilation, a number of olefin-terminated polyisobutylene prearms to SiH groups of a single cyclosiloxane molecule. In theory, the number of arms emanating from this molecule would total the number of SiH groups on the siloxane molecule. Thus, theoretically, for a hexamethylcyclosiloxane molecule (D.sub.6.sup.H) which has six SiH groups, a maximum of six polyisobutylene arms would radiate from the molecule.
In contrast, "higher order" star polymers have the potential for many more SiH groups for linking olefin-terminated polyisobutylene prearms to the core. While the number of arms emanating from the core cannot total more than the number of SiH groups originally available, the core in higher order star polymers is capable of having many times more SiH groups than are present on any one individual cyclosiloxane molecule. This is true because of the character of the core component in higher order stars which includes a plurality of individual cyclosiloxane molecules coupled together. Since only one reactive site is needed to couple two cyclosiloxanes, it will be appreciated that the number of SiH groups capable of linking polyisobutylene prearms to the core of the higher order star increases significantly where there are more SiH groups present. In other words, higher order stars might be envisioned as clusters of first-order star polymers linked together by their siloxane cores.
In the course of investigating the multi PIB-arm/cyclosiloxane core first order star polymers using D.sub.6.sup.H, it was observed that, in the presence of trace amounts of water in the hydrosilation charges, a small number of stars were observed which had a higher number of arms than the expected six. The formation of these "second-order" stars was explained by random core-core coupling, a process which has since been corroborated by direct experiments.
Since the number of common methylcyclosiloxanes with multiple SiH functions is quite limited (only two such cyclosiloxanes are commercially available, D.sub.4.sup.H and D.sub.5.sup.H), the designed preparation of "higher-order" multi-arm stars by hydrosilation of allyl-terminated PIB by a specific methylcyclosiloxane appeared to be cumbersome. Moreover, and potentially more detrimental, it was determined that steric compression severely limits the quantitative hydrosilation of multiple neighboring SiH groups. Thus, while the production of higher order stars by the random core-core couplings of a few multi-functional hydrogen cyclosiloxanes--that is, stars of two molecules of cyclosiloxane coupled together--was apparent from prior work, the controlled synthesis of stars of even higher orders, i.e., stars having three or more siloxanes coupled together, have heretofore been unknown.